Transient Solar Events and Their Effects on the Near-Earth Radiation Environment
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2021
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University of Hawaii at Manoa
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The Sun is comprised of complex, constantly changing plasma and magnetic field structures. Some of these structures can become unstable over time, leading to a number of different eruptive events. Most notable for this work are magnetic reconnection events in the solar corona. These events classically produce a solar flare and a coronal mass ejection (CME). In addition, particles may be accelerated to high energies and ejected from the Sun during these events, we call these particles Solar Energetic Particles (SEPs). As CMEs and SEPs propagate outward through the heliosphere, they have a number of interesting effects. CMEs can have significant effects on the Earth's magnetic field, called geomagnetic storms, and both CMEs and SEPs cause significant changes in the radiation environment. The ionizing radiation environment in space is normally dominated by galactic cosmic rays (GCRs), but an SEP event can suddenly cause a very large increase in ionizing radiation. This radiation poses as serious risk to humans and electronics in space, which makes it an important subject of study as humanity increases its presence in space. The shock wave in front of a CME as it propagates through the heliosphere may also accelerate some solar wind particles to higher energy, but the dominant effect of a CME on the radiation environment is to increase local modulation, reducing the flux of GCRs. This modulation of GCRs is called a Forbush decrease (FD). The Alpha Magnetic Spectromer (AMS) is a particle detector onboard the International Space Station (ISS). The detector measures cosmic rays (CR) in the rigidity range from 0.8 GV up to TV (~300 MeV/nuc into the TeV/nuc) with unprecedented precision. The upper rigidity range of the SEP spectrum can be measured by the lowest rigidity range of AMS. Part of this work is analysis to produce SEP spectra from AMS measurements. These comprise the most precise measurements of SEPs that have been made in this rigidity range. They reveal a continuum of qualitatively similar SEP events, with broad changes in quantitative properties. There is still some question as to how the amplitude of FDs depends upon the rigidity of GCRs modulated, and how this behavior is related to the properties of the CME and the solar wind. This work investigates this question by providing a method for defining FDs, and studying FDs using data from neutron monitors, AMS, and solar wind measurements.
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